1. Field of the Invention
The present invention relates to an optical waveguide and a fabricating (or manufacturing) method thereof, and an optical waveguide circuit, used in the fields of optical communication, optical signal processing, and optical measurement.
2. Description of the Related Art
Due to the worldwide spread of the Internet, optical communication systems using a WDM (wavelength-division multiplexing) technique or the like have spread commercially, in particular, into North America and the like. The WDM technique enables high-speed transmission of large amounts of data such as image data or video data.
Accordingly, research and development of lightwave (or optical) circuits constituting optical communication systems has been accelerated. In particular, waveguide-type lightwave circuits (i.e., optical waveguide (or wave-guiding) circuits), which can include optical waveguides formed on a single planar substrate by using the LSI fine-processing technique, have become the focus of attention because they have a high degree of integration and superior mass productivity, and accordingly, lightwave circuits having superior performance and a complicated structure can be realized using such optical waveguide circuits.
That is, optical waveguide circuits can provide various kinds of lightwave circuits by using functions of optical interference. In particular, optical wavelength-division multiplexing and demultiplexing devices are key devices in WDM systems.
FIG. 5A shows an arrayed-waveguide grating (AWG) type optical wavelength-division multiplexing and demultiplexing device as an example of the optical waveguide circuits. This AWG type optical wavelength-division multiplexing and demultiplexing device comprises input channel waveguides 1, output channel waveguides 2, a channel waveguide array 3, an input slab waveguide 4 for connecting the input channel waveguides 1 and the channel waveguide array 3, and an output slab waveguide 5 for connecting the output channel waveguides 2 and the channel waveguide array 3.
FIG. 5B shows an asymmetric Mach-Zehnder interferometer (MZI) type optical attenuator as another example of the optical waveguide circuits. In this device, two input waveguides 6, two output waveguides 7, and two arm waveguides 8 are connected with each other via two 3-dB directional couplers 9, and a thin-film heater type phase shifter 10 is formed on each arm waveguide 8.
FIG. 6 is a cross-sectional view of a conventional optical waveguide. On a silicon (Si) substrate 11, a lower cladding 12, a core 13, and an upper cladding 14 are formed.
Here, the polarization state of an optical signal passing through an optical network is not controlled; thus, the relevant optical waveguide circuit must have polarization-insensitive characteristics.
However, in the actual optical waveguide circuit, the core of each optical waveguide has geometrical birefringence or stress-induced birefringence, which causes polarization dependence. In particular, even if a silica-based optical waveguide circuit is employed and the core of each optical waveguide has an almost square cross-sectional shape (in this case, the geometrical birefringence can significantly be disregarded), the material and composition of the substrate generally differ from those of the waveguide portion; thus, various kinds of stress components are imposed on the core, and in most cases, stress imposed in the horizontal direction is not the same as that imposed in the vertical direction. As a result, due to photoelasticity, difference of the refractive index between the horizontal direction and the vertical direction (that is, stress-induced birefringence) occurs, thereby generating polarization dependence in the optical wave-guiding characteristics.
FIG. 3A is a graph showing an example of the optical transmitting characteristics of an AWG type optical wavelength-division multiplexing and demultiplexing device which is fabricated using silica-based glasses. FIG. 4A is a graph showing an example of the optical transmitting characteristics of an asymmetric MZI type attenuator which is also fabricated using silica-based glasses. As shown in each figure, in each example, a TM mode and a TE mode indicate different optical output characteristics, that is, polarization dependence is present.
In order to resolve such polarization dependence, a method of controlling the composition of the optical waveguide circuit (Reference 1:S. Suzuki et al., xe2x80x9cPolarization-Insensitive Arrayed-Waveguide Gratings Using Dopant-Rich Silica-Based Glass with Thermal Expansion Adjusted to Si Substratexe2x80x9d, Electronics Letters, Vol. 33, No. 13, pp. 1173-1174, 1997; and Reference 2:S. M. Ojha et al., xe2x80x9cSimple Method of Fabricating Polarization-Insensitive and Very Low Crosstalk AWG Grating Devicesxe2x80x9d, Electronics Letters, Vol. 34, pp. 78-79, 1998), and a method of inserting a wave plate (Reference 3:Y. Inoue et al., xe2x80x9cPolarization Mode Converter with Polyimide Half Waveplate in Silica-Based Planar Lightwave Circuitsxe2x80x9d, IEEE Photonics Technology Letters. Vol. 6, No. 5, pp. 175-177, 1994) are known.
The features of such conventional methods are shown in Table 1.
The method of controlling the composition has superior optical characteristics, productivity, and cost effectiveness; however, when this method is employed, the reliability of the device degrades. For example, when silica-based glasses are used, the stress inside the glass changes from compressive stress to tensile stress owing to the composition control. Therefore, the glass portion of the waveguide may easily have a crack or the like. Similarly, in an optical waveguide circuit using optical waveguides made of a material other than silica-based glasses (that is, a polymer or the like), the composition for realizing the polarization insensitivity does not always agree with the composition for obtaining the reliability of the device.
Currently, the method of inserting a wave plate is the leading method because the reliability of the device does not degrade. However, the power loss of the signal increases by approximately 0.5 to 1.0 dB, and generally, it is difficult to obtain preferable productivity and cost effectiveness in this method. This is because both the process of forming a groove for inserting a wave plate and the process of inserting the wave plate must be performed for each finished chip of the optical waveguide circuit.
As explained above, no currently-known method of realizing polarization insensitivity can satisfy both (i) the required characteristics related to the device, such as the optical characteristics and reliability, and (ii) the required productivity and cost effectiveness.
In consideration of the above circumstances, an object of the present invention is to provide an optical waveguide and an optical waveguide circuit having polarization insensitivity or a required low-level polarization dependence without degradation of the optical characteristics and reliability, and to provide a fabricating method of an optical waveguide and an optical waveguide circuit having polarization insensitivity or a required low-level polarization dependence without increasing the fabricating burden and the cost.
The above and other objects, and distinctive features of the present invention will be shown by the following explanations and attached drawings.
Therefore, the present invention provides an optical waveguide comprising:
a planar substrate;
a lower cladding which is provided on the planar substrate, where the lower cladding has a ridge;
a core, provided on the ridge of the lower cladding, for transmitting light; and
an upper cladding provided in a manner such that the core is covered with the upper cladding, and wherein:
the ridge has a shape predetermined so as to decrease polarization dependence of the optical waveguide to a required (or desired) level.
Preferably, the ridge has a shape predetermined so as to make the polarization dependence of the optical waveguide substantially zero.
In this case, typically, the height of the ridge is determined so as to satisfy the condition that with given geometrical birefringence B0 and photoelastic constants C1 and C2, a horizontal stress component "sgr"x imposed on the core from the upper and lower cladding and a vertical stress component "sgr"y imposed on the core from the upper and lower cladding have the following relationship:
"sgr"xxe2x88x92"sgr"y=B0/(C2-C1). 
It is also possible that the height of the ridge is determined so as to satisfy the condition that a horizontal stress component "sgr"x imposed on the core from the upper and lower cladding is substantially equal to a vertical stress component "sgr"y imposed on the core from the upper and lower cladding. Typically, the cross section of the core has a substantially square shape.
In the above basic structure, the planar substrate, the lower cladding, the core, and the upper cladding may have the following relationship:
xcfx81sub greater than xcfx81upper greater than xcfx81corexe2x89xa7xcfx81lower, or
xcfx81sub greater than xcfx81upper greater than xcfx81lower greater than xcfx81core 
where xcfx81sub, xcfx81lower, xcfx81core, and xcfx81upper are average thermal expansion coefficients of the planar substrate, the lower cladding, the core, and the upper cladding within the temperature range of 0xe2x89xa6Txe2x89xa6TS, TS being the softening temperature of the upper cladding.
As a typical example, the width of the ridge is equal to the width of the core. However, it is possible that the width of the ridge is not constant in the direction of its height.
Typically, the planar substrate is made of silicon, and the optical waveguide is made of silica-based glasses.
The present invention also provides an optical waveguide circuit having at least one optical waveguide as explained above. Typically, the optical waveguide circuit uses functions based on optical interference.
As a typical example, the optical waveguide circuit includes an arrayed-waveguide grating having a plurality of the optical waveguides, or an asymmetric Mach-Zehnder interferometer having a plurality of the optical waveguides.
The present invention also provides a method of fabricating an optical waveguide, comprising the steps of:
forming a lower cladding layer on a planar substrate;
forming a core layer on the lower cladding layer;
processing the core layer so as to make a core of the optical waveguide, wherein the lower cladding layer under the core layer is successively excavated to a predetermined depth so as to form a lower cladding and a ridge of the lower cladding of the optical waveguide; and
forming an upper cladding layer functioning as an upper cladding of the optical waveguide in a manner such that the processed core is covered with the upper cladding layer.
Preferably, in the step in which the ridge is formed, the ridge has a shape predetermined so as to make the polarization dependence of the optical waveguide substantially zero.
Here, typical examples relating to the shape of the processed ridge or the like are similar to those described above for the optical waveguide.
In the method of fabricating the optical waveguide as explained above, a core layer may be deposited and processed after the ridge of the lower cladding is formed. However, in this case, an additional processing step is necessary, and thus the burden on the processing increases. According to the fabricating method of the present invention, the ridge of the lower cladding can be simultaneously formed only by slightly increasing the time of etching performed when the core is processed.
The effects obtained by the disclosed present invention will be shown below:
(i) The stress imposed on the core can be changed, thereby realizing an optical waveguide circuit having polarization insensitivity or small polarization dependence with a required low level.
(ii) No additional fabricating process is necessary; thus, a polarization-insensitive type optical waveguide circuit can be realized without degradation of the device characteristics and without increasing the burden on the productivity and the cost.
(iii) The present invention can be applied to various kinds of optical waveguide circuits made by different fabrication methods, made of different materials, having different compositions, and having different specifications.
(iv) The present invention can be applied to various lightwave circuits such as wavelength-division multiplexing and demultiplexing devices, optical resonators, attenuators, thermo-optic switches using an asymmetric MZI, delay lines, and the like.
Here, Japanese Unexamined Patent Application, First Publication No. Hei 5-88037 titled xe2x80x9cGarnet Waveguide and Production Thereofxe2x80x9d discloses an example of a structure having a ridge under the core. As represented by this example, in the actual process of processing the core of the optical waveguide, the lower cladding layer may be slightly removed according to a design specification so that the core layer can be properly processed even if a processing error is present.
However, the object of the present invention for providing the optical waveguide is not to properly process the core. The present invention is to provide an optical waveguide having polarization-insensitive optical characteristics and an optical waveguide circuit using such optical waveguides based on the novel knowledge that the stress imposed on the core can be flexibly changed by providing a ridge under the core and suitably changing the height of the ridge. Therefore, the present invention is novel and the ridge in the conventional structure does not have a predetermined shape which would decrease the polarization dependence of the optical waveguide to a required (or desired) low level.
Japanese Unexamined Patent Application, First Publication No. Hei 8-160234 also discloses an optical wave-guiding element in which the cladding layer of a semiconductor optical wave-guiding element has a ridge structure. However, in this structure, owing to the stripe structure of an optical wave-guiding layer, geometrical birefringence is controlled and the polarization dependence of the optical wave-guiding layer is decreased. That is, in this case, the internal stress imposed from an embedded polyimide layer onto the core is not changed by providing a ridge structure in the cladding layer. That is, also in this structure, the ridge does not have a predetermined shape which would decrease the polarization dependence of the optical waveguide to a required low level, and thus the object, structure, and function of this conventional example differ from those of the present invention.